Z’ and leptoquark searches at the LHC

Quanyin Li

University of Science and Technology of China

2020.06.08

Conference on Flavour Physics and CP violation (FPCP) 2020

June. 08, 2020 2

Introduction to the LHC

Quanyin Li(USTC)

June. 08, 2020 3

Introduction to Z’

Quanyin Li(USTC)

ത𝑞

𝑞

𝑒−

𝑒+

𝑍′ത𝑞

𝑞

μ−

μ+

𝑍′ ത𝑞

𝑞

τ−

τ+

𝑍′

ത𝑞

𝑞

𝑒−

μ+

𝑍′ ത𝑞

𝑞

𝑒−

τ+

𝑍′ത𝑞

𝑞

μ−

τ+

𝑍′

ത𝑞

𝑞

𝑞𝑍′

ത𝑞

Z’ is a new gauge boson which is

predicted in many models.

Sequential Standard model(𝑍𝑆𝑆𝑀′ ), E6-

motivated Grand Unification

model(𝑍ψ′ 𝑎𝑛𝑑 𝑍χ

′ ) and Dark-Matter

Mediator model(𝑍𝐷𝑀′ ).

Include hadronic decay and leptonic

decay.

Leptonic decay with both same flavor and

different flavor lepton pairs.

Searches in different channels are

performed.

List of Z’ searches

June. 08, 2020 4Quanyin Li(USTC)

• ee and mumu channel:

ATLAS: 13 TeV 139 fb-1 Physics Letters B 796 (2019) 68–87

CMS: 13 TeV 36 fb-1 JHEP06(2018)120

• tautau channel:

ATLAS: 13 TeV 36.1 fb-1 JHEP01(2018)055

• emu, etau and mutau channel:

ATLAS: 13 TeV 36.1 fb-1 PHYSICAL REVIEW D 98, 092008 (2018)

CMS(only emu): 13 TeV 35.9 fb-1 JHEP04(2018)073

• dijet:

ATLAS: 13 TeV 139 fb-1 JHEP03(2020)145

CMS: 13 TeV 137fb-1 JHEP05(2020)033

ATLAS: dijet+lepton 13 TeV 139fb-1 submitted to JHEP

• other channels:

ATLAS: ttbar 13 TeV 139 fb-1 submitted to JHEP

June. 08, 2020 5

Z’ to dielectron/dimuon @ATLAS

Quanyin Li(USTC)

Several improvements compare with 36.1 fb-1:

• Higher luminosity

• The reconstruction software

• The first time of using data-driven fit.

Event selection:

• Mll > 225 GeV

• ee channel: Two electrons.

• μμ channel: Two muons with opposite charge.

New background estimation method: data-driven fit

• Fit function:

139 fb-1

June. 08, 2020 6

Z’ to dielectron/dimuon @CMS

Quanyin Li(USTC)

Dielectron Invariant Mass

of CMS result

Dimuon Invariant Mass

of CMS result

36 fb-1

June. 08, 2020 7

Z’ to tautau @ATLAS

Quanyin Li(USTC)

Two channels:

• τlepτhad: At least one τhad-vis and exactly one lepton, mT(lep, 𝐸𝑇𝑚𝑖𝑠𝑠) < 40 GeV

• τhadτhad: At least two τhad-vis, no electrons or muons, |ΔΦ(τ1, τ2)|>2.7

Mass definition:

The latest CMS Z’tautau:13 TeV 2.2 fb-1, JHEP02(2017)048

36.1 fb-1

June. 08, 2020 8

Z’ to dilepton with different flavor @ATLAS

Quanyin Li(USTC)

Include eμ, eτ, μτ channels.

Data-driven method for

fake lepton background.

Hadronic decay of τ-leptons.

Only events with exactly 2

different-flavor leptons.

36.1 fb-1

June. 08, 2020 9

Z’ to emu @CMS

Quanyin Li(USTC)

Only eμ channel

Four events observed in the

region meμ > 1.5 TeV

Lower limits for Z’: 4.4 TeV

35.9 fb-1

June. 08, 2020 10

Z’ to dijet @ATLAS

Quanyin Li(USTC)

Data-driven fit used in background estimation.

New b-tagging algorithm: DL1r b-tagging

Event selection category:

2.9 TeV

139 fb-1

June. 08, 2020 11

Z’ to dijet @CMS

Quanyin Li(USTC)

Data-driven fit used in background estimation.

137 fb-1

June. 08, 2020 12

Z’ in association with V @ATLAS

Quanyin Li(USTC)

Event selection in signal region:

• At least one lepton(electron/muon)

• At least two jets

• Mjj >= 220 GeV

Data-driven fit used in background estimation.

139 fb-1

June. 08, 2020 13

Z’ to ttbar @ATLAS

Quanyin Li(USTC)

ttbar events with fully hadronic decay:

• >=2 jets with >= 1 large-R jet

• Mjj (2 large-R jets with highest-pT) > 1.4 TeV

• |ΔΦjj| > 1.6.

• SR1b: exactly 1 of the 2 top-tagged jets is a b-tagged jet

• SR2b: both the 2 top-tagged jets are b-tagged jets

Background estimation:

• First using data-driven fit:

• New top-tagging to improve the sensitivity

139 fb-1

June. 08, 2020 14

Introduction of leptoquark

Quanyin Li(USTC)

A common feature of many new physics models is the

presence of a new class of bosons, called leptoquarks

(LQs), that carry both lepton (L) and baryon numbers (B).

Include pair production and single production.

LQs has 3 generations like leptons and quarks.

Fractional charge.

Decays to lepton + quark in the same generation with

LQs.

Decays to lepton + quark in the different generation with

LQs.

LQs can provide an explanation of the flavour anomalies

seen at B factories

Pair production of LQs.

Single production of LQs.

List of leptoquark searches

June. 08, 2020 15Quanyin Li(USTC)

Leptonquark searches at the LHC:

• First and second generations:

ATLAS: lljj and lνjj @13 TeV 36.1 fb-1 Eur. Phys. J. C (2019) 79:733

CMS: eejj and eνjj @13 TeV 35.9 fb-1 PHYSICAL REVIEW D 99, 052002 (2019)

CMS: μμjj and μνjj @13 TeV 35.9 fb-1 PHYSICAL REVIEW D 99, 032014 (2019)

• Third generation:

ATLAS: 5 channels @13 TeV 36.1 fb-1 JHEP06(2019)144

CMS: ττjj @13 TeV 35.9 fb-1 JHEP03(2019)170

CMS: ttττ @13 TeV 35.9 fb-1 Eur. Phys. J. C (2018) 78:707

CMS: τb @13 TeV 35.9 fb-1 JHEP07(2018)115

CMS: Quark+Neutrino @13 TeV 35.9 fb-1 PHYSICAL REVIEW D 98, 032005 (2018)

• Different/mixed generations:

CMS: ttμμ @13 TeV 35.9 fb-1 PHYSICAL REVIEW LETTERS 121, 241802 (2018)

June. 08, 2020 16

First and second generation LQ @ATLAS

Quanyin Li(USTC)

Event selection:

• lljj: At least 2 jets and 2 same-flavor leptons

• lνjj: At least 2 jets, 1 lepton and a missing transverse energy (> 40 GeV)

eejj, μμjj, eνjj and μνjj.

β = 0.5, Observed limits 1290 GeV

β = 0.5, Observed limits 1230 GeV

Details shown in talk: “Search for

LQs with the ATLAS detector”.

36.1 fb-1

June. 08, 2020 17

First generation LQ @CMS

Quanyin Li(USTC)

• Two event selection criteria: eejj and eνjj

• The branching fractions for the LQ decay

are expressed in terms of a free parameter

β, where β denotes the branching fraction

to an electron and a quark, and 1 − β the

branching fraction to a neutrino and a

quark.

• β = 1.0: eejj limit 1435 GeV

• β = 0.5: eνjj limit 1195 GeV

eejj eνjj

35.9 fb-1

June. 08, 2020 18

Second generation LQ @CMS

Quanyin Li(USTC)

• Two event selection criteria: μμjj and μνjj

• The branching fractions for the LQ decay

are expressed in terms of a free parameter

β, where β denotes the branching fraction

to an electron and a quark, and 1 − β the

branching fraction to a neutrino and a

quark.

• β = 1.0: μμjj limit 1530 GeV

• β = 0.5: μνjj limit 1150 GeV

35.9 fb-1

June. 08, 2020 19

Third generation LQ @ATLAS

Quanyin Li(USTC)

Channels Event Selection Covered Decay Mode

bτbτ 1. 1 e/μ, 1 τhad, >=2(>=1 b) jets

2. 0 e/μ, 2 τhad, >=2(>=1 b) jets

3. No loose lepton veto, 2 τhad, >=2(>=1 b) jets

LQLQ bτbτ

LQLQ tτtτ

tt+𝐸𝑇𝑚𝑖𝑠𝑠(1 lepton) >= 4 jets with >=1 b jet, 1 e/μ, high 𝐸𝑇

𝑚𝑖𝑠𝑠 LQLQ tνtν

tt+𝐸𝑇𝑚𝑖𝑠𝑠(0 lepton) >= 4 jets with >=1 b jet, 𝐸𝑇

𝑚𝑖𝑠𝑠 > 250 GeV LQLQ tνtν

LQLQ bνbν

ττb+𝐸𝑇𝑚𝑖𝑠𝑠 1. 1 e/μ, 1 τhad, >=2 (>=1 b ) jets

2. 2 τhad, >=2(>=1 b) jets

LQLQ tν/bτ

LQLQ bν/tτ

bb+𝐸𝑇𝑚𝑖𝑠𝑠 1. 2 b jets, 0 leptons and large 𝐸𝑇

𝑚𝑖𝑠𝑠

2. 2 b jets, 1 leptons and large 𝐸𝑇𝑚𝑖𝑠𝑠

LQLQ bνbν

Details shown in talk: “Search

for LQs with the ATLAS

detector”.

36.1 fb-1

June. 08, 2020 20

Third generation LQ (ττjj) @CMS

Quanyin Li(USTC)

Event selection:

• At least 2 τh candidates.

• At least 2 jets.

LQs with a mass less than 1.02TeV are excluded at 95%

confidence level.

35.9 fb-1

June. 08, 2020 21

Third generation LQ (ttττ) @CMS

Quanyin Li(USTC)

Event selection:

• Exactly one isolated muon or electron

• At least one τh lepton

• 𝑝𝑇𝑚𝑖𝑠𝑠 > 50 𝐺𝑒𝑉

• Category A: lτh+jets

• Category B: lτhτh+jets

Final event yield in category B in the muon and electron

channels for different leptoquark mass hypotheses

The scalar LQ3s

are excluded with

masses below 900

GeV, for B = 1

35.9 fb-1

June. 08, 2020 22

Single third generation LQ (τb) @CMS

Quanyin Li(USTC)

Three channels:

• τhτh: 2 τh candidates in addition to a b-tagged jet

• μτh: 1 muon, 1 τh candidate and 1 b-tagged jet

• eτh: 1 electron, 1 τh candidate and 1 b-tagged jet

λ=1: limit of mass > 740 GeV

λ=2.5: limit of mass > 1050 GeV

35.9 fb-1

June. 08, 2020 23

All generation LQ (quark+neutrino) @CMS

Quanyin Li(USTC)

Event selection:

• Nj >=2

• MT2 > 200 GeV(HT<1500 GeV)

• MT2 > 400 GeV(HT>1500 GeV)

• Categorized according to HT

(scalar sum of jet pT), MT2

(computed from jets and the

Ԧ𝑝𝑇𝑚𝑖𝑠𝑠), Nj (number of jet) and

Nb (number of b jet).

κ is a dimensionless coupling that is 1 in the Yang-Mills case and 0 in the minimal coupling case.

35.9 fb-1

June. 08, 2020 24

Mixed generation LQ @CMS

Quanyin Li(USTC)

Decay mode: LQ tμ

Event selection:

• At least two muons

• At least two jets

• At least one jet must be b-tagged.

• Mμμ - MZ > 20 GeV

• Category A: at least 3 letpons with at least 2

opposite charged muons

(leptonic top quark)

• Category B: All the remaining events.

S𝑇𝑙𝑒𝑝

is the scalar pT sum of all selected muons and electrons and

ST is defined as the scalar sum of S𝑇𝑙𝑒𝑝

, 𝑝𝑇𝑚𝑖𝑠𝑠, and the pT of all

selected jets.

35.9 fb-1

June. 08, 2020 25

Summary

Quanyin Li(USTC)

No evidence for New Physics yet.

Limits are set for different theory models.

New searches are being performed to look for Z’ and LQs with LHC full

run II data.

Thanks!ATLAS publication list CMS publication list

June. 08, 2020 26

Backup: summary of ATLAS Z’(jet) result

Quanyin Li(USTC)

June. 08, 2020 27

Z’ to ditau @CMS

Quanyin Li(USTC)

Four channels:

• τeτμ：DY+ttbar 75%

• τeτh: DY+Wjet 70%

Mass definition and signal region:

• τμτh: DY+Wjet 90%

• τhτh: QCD 80%

June. 08, 2020 28

Z’ to dijet @ATLAS

Quanyin Li(USTC)

June. 08, 2020 29

Third generation LQ @ATLAS

Quanyin Li(USTC)

bτbτ: 1030 GeV for LQu3 and 930 GeV for LQd

3 tt+𝐸𝑇𝑚𝑖𝑠𝑠(1 lepton): 930 GeV

tt+𝐸𝑇𝑚𝑖𝑠𝑠(0 lepton): 1000 GeV for LQu

3 and 860 GeV for LQd3 ττb+𝐸𝑇

𝑚𝑖𝑠𝑠: 780 GeV for LQu3 and 800 GeV for LQd

3

bb+𝐸𝑇𝑚𝑖𝑠𝑠: 970 GeV